Fuel cells have been in the transition from the fuel cells of first generation which use a liquid substrate such as a phosphoric acid aqueous solution or fused carbonate for the fuel cells of second generation such as polymer electrolyte type fuel cells (hereinbelow, referred to as PEFC) or solid-oxide electrolyte type fuel cells (hereinbelow, referred to as SOFC). Among these fuel cells of second generation, PEFCs which use a fluorine or hydrocarbon type polymer electrolyte film as the electrolyte, have many disadvantages that the fuel used is limited to pure hydrogen (H2); temperature control of very narrow range (from 65° C. to 85° C.) is required; a delicate control is necessary to determine the water (H2O) content in the electrolyte film; a large quantity of expensive platinum catalyst has to be used; the durability is only about from 2,000 to 3,000 hours, and freezing should be avoided, consequently, they can not be used in a cold district because there is a danger of freezing. Further, since the fuel used is limited to pure hydrogen, it is necessary to restructure entirely the existent social infrastructure (e.g., gas, LPG, petroleum etc.) to be adapted to a hydrogen system, which requires a huge capital investment for social infrastructure. Cost for the restructuring would be added to cost for producing hydrogen, so that the unit price per unit calorific power will increase.
In addition, there are disadvantages that since the working temperature is low and therefor, the exhaust temperature is low, it is difficult to recover effectively exhaust heat in, for example, a co-generation system, and if the demand of heat is higher than the demand of electric power, a combustion aid using an expensive hydrogen fuel is needed, and accordingly, they can not always be the optimum energy-saving system from the viewpoint of heat balance.
Further, since PEFC requires expensive pure hydrogen fuel, it would be difficult to assert the advantage in using them in comparison with existent heat engines, hybrid cars etc. when efficiency including hydrogen production cost is considered.
On the other hand, SOFC allows using various kinds of hydrocarbon type fuel other than pure hydrogen and has the highest theoretical power generation efficiency among all kinds of fuel cells. However, since the conventional SOFC employed yttria-stabilized zirconia (YSZ) as its electrolyte, it was necessary to operate it at a high temperature of about 1,000° C., and therefor, ceramic having heat resisting properties was used for not only the fuel cell body but also main parts of the structural body. Accordingly, it was necessary to start it with a spending time from several hours to ten and several hours in order to prevent the breakage due to a thermal stress generated by an ununiform distribution of temperature. Further, it was unable to follow a rapid load change.
In recent years, a solid-oxide material such as scandium-stabilized zirconia (ScSZ) or lanthanum gallate type solid-oxide (LSGM), which has the same oxygen ion conductivity at from 650 to 800° C. as YSZ of 1,000° C., has been developed. Accordingly, a metal material can be used for many parts of the structural body, and flexibility in structure designs has remarkably been improved. However, the SOFC's main body was of the structure comprising cylindrical tubs or stacked circular disks. Accordingly, it had the disadvantages that a thermal stress generated in each part of the structural body could not sufficiently be reduced, and the starting time could not sufficiently be shortened.
Further, it was necessary for the conventional SOFC to keep the temperature inside the system uniformly to be in a high temperature state of about 1,000° C. in order to maintain power generation performance and to prevent the breakage due to a thermal stress applied to the materials and the structural body. Accordingly, it was necessary to suck exhaust gas and exhaust air through venturis or the like, the sucked gases being mixed and the gas mixture being forcibly circulated. Accordingly, the fuel and oxygen were supplied with low concentrations, whereby an output power per unit area of the fuel cell could not be increased.
Detailed description will be made on the structure and function of the system of a conventional typical SOFC employing a cylindrical cell unit, with reference to FIG. 7.
Fuel is supplied from a fuel inlet 61 and is passed through a cylindrical interior reforming device 62, meanwhile the temperature of the fuel is elevated. The fuel is reversed at an end portion to flow in a direction of arrow mark 63 along a fuel pole. Fuel exhaust gas containing unreacted fuel is passed through a returning line 64, and sucked by a fuel inlet venturi 65 to be returned to a fuel supply line. The surplus fuel exhaust gas is supplied to a combustor 74 through a fuel exhaust pipe 66.
On the other hand, air as an oxidizing agent is supplied from an air inlet 77 and flows through an air header 68 in a direction of arrow mark 69, meanwhile it is preheated. The air is reversed at an end of the cell unit to flow in a direction of an air pole 70 to thereby supply oxygen to the fuel cell. The air which has supplied oxygen to the fuel cell flows in an air exhaust pipe in a direction of arrow mark 71, and sucked by an air inlet venturi 72 to be returned to an air supply line. The surplus air flows in an exhaust pipe in a direction of arrow mark 73 and is mixed with the fuel exhaust gas in the combustor 74 for combustion, and the air after the combustion is discharged from an exhaust port 76 through an exhaust pipe 75.
In the conventional SOFC having the above-mentioned structure, it was necessary to elevate gradually the temperature from a cold state to a high-temperature state capable of generating power while the fuel gas and air were forcibly circulated, and the temperature of the entire cell unit was kept uniform to avoid the breakage due to a thermal stress. Accordingly, it took much time for warming-up from the starting of the apparatus to a full power state, thus, there was a drawback of being inconvenient.
Further, the conventional SOFC had such structure that the exhaust at the fuel supply side and the exhaust at the air supply side were sucked by using venturis, the sucked gases were mixed with supplied gases, the gas mixture is forcibly circulated in a thinned state, and in addition, the surplus exhaust fuel and the surplus exhaust air were mixed for combustion, and the combustion gas was discharged into atmosphere. Accordingly, the exhaust contains large quantities of unreacted fuel and unreacted oxygen, and therefor, sufficient power generation efficiency could not be expected.
On the other hand, JP-A-10-189023 and JP-A-11-297343 disclose fuel cells of honeycomb structure employing a solid oxide, as examples.
The fuel cells disclosed therein have a honeycomb structural body employing yttria-stabilized zirconia (YSZ) or scandium-stabilized zirconia (ScSZ) as a solid oxide. However, the disclosed fuel cells belong a honeycomb type fuel cell in which fuel pole cells and air pole cells adjacent to each other are arranged alternately in a form of so-called checker board design, and no means for removing heat generated in its honeycomb cells is provided. Accordingly, heat built up in the honeycomb to elevate the temperature in the honeycomb, and on the other hand, the outer peripheral portion of the honeycomb was cooled by heat radiation and cooldown, with the result that a large temperature difference was resulted between the central portion and the outer peripheral portion of the honeycomb, whereby the breakage of the honeycomb was caused due to a thermal stress by the temperature difference.
Accordingly, these fuel cells had such structure that in the same manner as the conventional structure shown in FIG. 7, the exhaust of fuel gas and exhaust air were sucked with venturis to mix them with supplied gases, and thinned gases as the result of mixing were circulated forcibly so that no large thermal stress was resulted in the honeycomb cells constituting the fuel cells. Further, in these fuel cells, it was necessary to elevate gradually the temperature with a sufficient warming-up time after the starting of the apparatus to a normal operation, hence, there was a drawback of being inconvenient.
Further, since the exhaust fuel gas and exhaust air were sucked for mixing by using the venturis and the gas mixture was circulated forcibly, fuel in the circulated fuel gas and oxygen in the circulated air were thinned and their concentrations were decreased. Further, since the concentration of unreacted fuel gas and the concentration of unreacted oxygen contained in the exhaust gas also were high, whereby the power generation efficiency could not sufficiently be increased. Further, the output density per unit surface area could not be increased.
The present invention is to solve the above-mentioned disadvantages and to provide a SOFC type fuel cell being small in size, large in output, having good efficiency and being excellent in starting characteristics and load variation characteristics, by cooling uniformly the inner portion of the honeycomb structural body as that a temperature difference at inner and outer portions of the fuel cell can be reduced to thereby prevent the generation of a thermal stress, by supplying, to the fuel cell, fuel and air of high concentration without being thinned, and by utilizing remaining unreacted fuel in fuel exhaust gas at the fuel cell outlet and the remaining unreacted oxygen in exhaust air to generate power until the fuel and the oxygen reach sufficiently low concentrations.